1. Field of the Invention
The present invention relates to a Mg--Ca sacrificial anode, and in particular to a Mg--Ca sacrificial anode which is capable of restricting fire which frequently occurs during a melting of Mg, and for providing characteristics equal to or better than the conventional Mg alloy sacrificial anode in a driving potential and efficiency.
2. Description of the Background Art
A Mg-group sacrificial anode is connected to a steel structure (pipe, tank, etc.), which is buried underground, by an electric cable and is used to prevent corrosion of the steel structure. This is known as an internal power supply type (or sacrificial anode type) corrosion resisting technique (Contrary thereto, there is an external power supply type which supplies DC power).
The sacrificial anode represents a metal which is sacrificially corroded to prevent any corrosion of the surrounding metals. For example, there is a zinc coated steel in which zinc is coated on a steel. When the zinc coated steel is exposed under a corrosion environment, a galvanic cell is formed so that the zinc is priorly corroded and becomes an anode, and the steel becomes a cathode for thereby preventing corrosion.
In the cathodic protection method which is used for ship and marine structure, Al or Zn alloy is used as a sacrificial anode. The Mg-based sacrificial anode is generally used for a steel structure buried in soils which have a high resistivity. Since Mg has Inherent negative potential and high electrochemical equivalents, Mg is one of the most effective and economical metals for underground sacrificial anodes.
In order to effectively use the sacrificial anode to prevent any corrosion of a steel structure which is buried underground, a driving potential and anode efficiency of the sacrificial anode should be high for the reason that it is possible to prevent corrosion of steel by supplying sufficiently high protection current to the steel structure compared to an IR drop. The anode efficiency represents a ratio of total current amount which is produced by unit mass of an anode as a result of anodic dissolution, with respect to the theoretical current amount which can be calculated by Faraday's Law, and it is directly related to the life span of the sacrificial anode.
Before and after the world war II, Dow Chemical Company developed a sacrificial anode formed of a highly pure Mg, an AZ63 alloy having an excellent anode efficiency, and a Mg--Mn alloy having a high driving potential. In many countries, the Mg-group sacrificial anode is standardized with respect to the above-described various alloys. As seen in the Table 1, although a pure Mg sacrificial anode has a low open circuit potential value of -1.62.about.-1.67V based on a saturated copper/copper sulfute reference electrode and an anode efficiency characteristic of 40 to 50%, the above-described characteristic is observed only in the case of high purity of above 99.95% (The electrochemical potential (V) value hereinafter is a value determined based on a reference of the saturated copper/copper sulfate electrode). Therefore, in order to limit the impurities, a Mg--Mn alloy having a small amount of Mn was disclosed. This alloy is generally used under an environment in which the resistivity is high, the open-circuit potential is -1.75V, and the anode efficiency is 50%. The AZ63 alloy is manufactured by significantly enhancing the electrochemical characteristic by adding an alloy element such as Al, Zn, etc, by a fixed ratio and is generally used under an environment in which the resistivity is low, the open-circuit potential is -1.55V, and the efficiency is 60%. As another example, a study shows that the efficiency characteristic is more than 20% by heat-treating Mg--Mn alloy or AZ63 alloy or adding a low melting point element such as Be, etc.
Generally, a small amount of Mn is contained in the Mg-based sacrificial anode because Mn is an excellent scavenger element in Mg-alloy for control of the effects of impurities. Mn is added using a metal Mn or MnCl.sub.2 when manufacturing the Mg--Mn alloy or the AZ63 alloy. In the case of using a metal Mn, since the melting point of the metal Mn is 1244.degree. C., a dissolution temperature is increased for expediting a dissolution of the metal Mn into the Mg-alloy and the metal should be also finely ground in order to increase the producing yield of the Mn so that the fabrication process is complicated. In the case of using MnCl.sub.2, and since MnCl.sub.2 exists where the MnCl.sub.2 includes four waters of crystallization at room temperature, a thermal dissolution process should be performed in order to eliminate the waters of crystallization at a certain temperature of above 195.degree. C. In addition, if the Mg alloy which is used as a conventional sacrificial anode is exposed to atmosphere during a casting of the Mg, fire may occur thereby generating a large amount of sludge and a certain bad smell resulting in a large loss of source materials and degrading the working environment.